编译报错：
undefined reference to `cv::imshow(cv::String const&, cv::_InputArray const&)'
解决办法：
中加入
find_package(OpenCV 3 QUIET)
target_link_libraries(program_name ${OpenCV_LIBS})
OpenCV 3 是安装OpenCV的版本号，如果装的4.*就写OpenCV 4
program_name是可执行程序名，即add_executable(program_name, code_file)中的program_name